It is said that locomotion of a living organism such as an animal and a human being is generated and controlled by a pattern generator, that is, a neural oscillator CPG (Central Pattern Generator) network that is said to exist in a spinal cord. Hereinafter, a plurality of nonlinear oscillators included in the CPG network are referred to as the CPG in order that the CPG network and the CPG are distinguished from each other.
Many models such as a Matsuoka model have been proposed as a CPG network model, and the CPG network model is applied to real machines (robots) (see Non-Patent Documents 1, 2, and 3). Taga shows that the CPG network and a masculo-skeletal system generate mutual entrainment through sensory input, and Taga performs a simulation of two-legged locomotion. Kimura expands the Taga model to produce a quadruped locomotion robot and realizes some locomotion through irregular terrain by utilizing an ability of entrainment with the CPG network and musclo-skeletal system. As described above, the CPG network is a pattern generator that can explain abilities of irregular ground locomotion and the like which are possessed by a living organism.
The conventional CPG network model is one that simulates a structure and an activity of a nerve. In the conventional CPG network model, the CPGs are mutually connected through connection weights, and the nerve indicating a fatigue degree or the nerve indicating an internal state is designed in one CPG.
On the other hand, a Phaselock Techniques is known as a phase control method in which a phase can be controlled without use of the mutual connection between oscillators. A Phase-Locked Loop (PLL) is a specific example of the Phaselock Techniques and consists of a Phase Detector (PD), a Lowpass Filter (LF), and a Voltage Controlled Oscillator (VCO). A phase difference between two signals is detected in the PD, and a period of the oscillator is controlled in the VCO if needed. Assuming that a control signal is a signal outputted from the VCO while a target signal is a signal that becomes a target, in an operating principle of the PLL, the phase difference between the control signal and the target signal is detected by the PD, the output signal of the PD is inputted to the VCO through the LF, and the period of the control signal is controlled by the VCO until the value of the VCO becomes zero.
Volkovskii et al. propose a CPG network model in which the PLL is used (see Non-Patent Document 4). This CPG network model is not an oscillating system which has a limit cycle, because a sine function is used as the CPG model. Hoppensteadt et al. propose a PLL network model (see Non-Patent Document 5). In this model, a sinusoidal function is used as the VCO, and a model concerning pattern recognition is proposed as application of the PLL network model.
A Van der Pol (VDP) equation is a mathematical model that can describe the theory of vacuum-tube oscillators (see Non-Patent Document 6). There have been proposed some CPG networks in which the VDP equation is used (see Non-Patent Document 7).    [Non-Patent Document 1] K. Matsuoka, “The dynamic model of binocular rivalry”, Biological Cybernetics, Vol. 49, pp. 201-208, 1984.    [Non-Patent Document 2] G. Taga and two others, “Self-organized control of bipedal locomotion by neural oscillators in unpredictable environment”, Biological Cybernetics, Vol. 65, pp. 147-159, 1991.    [Non-Patent Document 3] H. Kimura and two others, “Adaptive Dynamic Walking of a Quadruped Robot on Natural Ground Based on Biological Concepts”, International Journal of Robotics Research, Vol. 26, pp. 475-490, 2007.    [Non-Patent Document 4] A Volkovskii and five others, “Analog electronic model of the lobster pyloric central pattern generator”, Journal of Physics, Conference Series, Vol. 23, pp. 47-57, 2005.    [Non-Patent Document 5] Frank C. Hoppensteadt and one other, “Pattern Recognition Via Synchronization in Phase-Locked Loop Neural Networks”, IEEE Transactions on neural networks, Vol. 11, No. 3, 2000.    [Non-Patent Document 6] Van der Pol, “On relaxation oscillations”, Phil. Mag., No. 2, pp. 987-993, 1926.    [Non-Patent Document 7] Max S. Dutra and two others, “Modeling of a bipedal locomotor using coupled nonlinear oscillators of Van der Pol”, Biological Cybernetics, No. 88, pp. 286-292, 2003.